High-resolution scanning apparatus

ABSTRACT

The high-resolution scanning apparatus utilizes conventional transport mechanisms in combination with a precision interferometric distance-measuring apparatus to achieve extremely-precise measurements of pixel locations thereby permitting scanning resolutions exceeding 10,000 dpi to be achieved. The high-resolution scanning apparatus is comprised of the conventional elements of a scanner together with one or more precision interferometric distance-measuring devices for the purpose of establishing the precise location of the pixel being measured. With this approach the mechanical limitations of the transport mechanisms that cause the optical system to scan the image-containing medium do not limit the resolution achievable with the scanning apparatus. The invention also provides compensating unit for variations in the object distance to the lens introduced by imperfections in the transport mechanisms. The light detectors are mounted on a platform that can be moved either nearer to or farther from the lens in accordance with changes in the object distance as measured by a precision interferometric distance-measuring device.

BACKGROUND OF INVENTION

The high-resolution scanning apparatus that constitutes the presentinvention relates generally to methods and apparatus for convertingimages recorded on paper and film into digital data so that the imagescan be conveniently communicated to other locations and processed bydigital computers. More specifically, the high-resolution scanningapparatus has to do with mechanisms that systematically scan animage-containing medium thereby enabling electronic circuits to extractthe information content of the medium by measuring either thereflectivity or the transmissivity of the medium as a function of thetwo coordinates that identify points in the plane of the medium.

The development of personal computers with the computational power toprocess imagery has stimulated a demand on the part of illustrators,photographers, graphic artists, and others whose work is benefited bythe computer manipulation of imagery for scanning apparatus that canconvert the imagery of photographs and transparencies into digital datathat can be handled by computers.

Scanning apparatus consists of a source of light which illuminates theimage-containing medium, an optical system that images picture elements("pixels") of the medium on light detectors, and a transport mechanismwhich moves the light source, the optical system and the light detectorsalong a predetermined path thereby obtaining measures of reflectivity ortransmissivity of the medium throughout the area of the medium.

A performance parameter of considerable interest is the scannerresolution--how far apart must the pixels be for the scanner to measurethe light emanating from a particular pixel. The fundamental limitationof resolution in present-day scanners is the precision of the transportmechanism. Typically, the transport mechanism in today's scannersconsists of a screw- or belt-driven carriage moving along a guide rail.The driving means is quite often a stepping motor. The position of thecarriage along the rail is obtained by counting the number of steps bythe stepping motor.

The precision with which the position of a platform can be establishedin transport mechanisms of this type is limited by frictional effects,temperature effects, the difficulties of manufacturing ultra-preciseparts, and material properties. These limitations appear to limit theresolution of scanners that utilize this type of transport mechanism tosomething like 2000 dpi (dots per inch).

Thus, while the realizable resolution of photographic materials can be10,000 dpi or more, the resolution of the great majority of scannerspresently on the market is less than 2000 dpi. The efficiency andeconomy of computer processing of imagery is thereby not available tothose who wish to retain the high resolution of their photographs andtransparencies during processing.

For example, there exists a large volume of data that has been archivedon film and requires computer processing for publishing, electronicaccess and research. Much of this historical film has been reduced up to52 times and to properly recover this information in a digital format,the scanner must have a resolution of at least 10,000 dpi.

The demonstrated usefulness of computer processing of imagery, theavailability of high-resolution of photographic materials, and thegrowing population of graphic artists who wish to optimally meld thephotographic and electronic technologies together for their benefit andthe benefit of their clients has created a need for reasonably-pricedscanning apparatus with resolutions of 10,000 dpi or more.

BRIEF SUMMARY OF INVENTION

The high-resolution scanning apparatus utilizes conventional transportmechanisms in combination with a precision interferometricdistance-measuring apparatus to achieve extremely-precise measurementsof pixel locations thereby permitting scanning resolutions exceeding10,000 dpi to be achieved.

The high-resolution scanning apparatus is comprised of the conventionalelements of a scanner together with one or more precisioninterferometric distance-measuring devices for the purpose ofestablishing the precise location of the pixel being measured. With thisapproach the mechanical limitations of the transport mechanisms thatcause the optical system to scan the image-containing medium do notlimit the resolution achievable with the scanning apparatus.

The present invention also envisions a mechanism for tilting the opticalsystem relative to the transport mechanism on which it is mounted withthe tilting mechanism servomechanically controlled by thedistance-measuring devices. This approach permits the optical system tobe moved to the approximate location of the pixels to be measured andthen pointed in the direction required for measuring the light emanatingfrom a particular pixel or group of pixels. The tilting mechanismrepresents a "fine" adjustment superimposed on the "coarse" adjustmentprovided by the conventional transport mechanisms. Since the tilting ofthe optical system can be limited to very small angles, the tilting canbe accomplished rapidly and precisely.

The invention may utilize a plurality of light detectors whichsimultaneously measure the light emanating from an equal number ofpixels, the pixels being imaged on the light detectors on a one-to-onebasis. The invention has the capability of measuring the location ofsome arbitrarily-selected pixel--the "reference" pixel--and determiningthe locations of all other pixels based on the reference pixelmeasurement.

The illuminating source of light must be above and the optical systembelow the medium when the medium is a transparency. The inventionprovides for the closely-synchronized movement of two platforms alongparallel tracks, one carrying the light source on the track above themedium and the other carrying the optical system on the track below themedium.

It is intended that the present invention be capable of scanningimage-containing media that are in color. This task can be accomplishedin the present invention with a color-corrected diffraction-limitedlens. A less-expensive approach, and therefore a more desirable one, isto use a non-color-corrected, diffraction-limited lens and eitherilluminate the medium sequentially with narrowband light sources ofdifferent colors or illuminate with white light and filter the lightinto narrow color bands at the detectors, particular detectors beingdedicated to each of the color bands. Given a fixed object distance, theimage distance of a non-color-corrected lens varies with wavelength. Byplacing the groups of detectors at the image distance corresponding tothe light color to which they respond, performance equivalent to thatprovided by a color-corrected lens can be obtained.

The invention also provides a means for compensating for variations inthe object distance introduced by imperfections in the transportmechanisms. The light detectors are mounted on a platform that can bemoved either nearer to or farther from the lens in accordance withchanges in the object distance as measured by a precisioninterferometric distance-measuring device.

One of the objects of the invention is to provide a means for scanninghigh-resolution photographic media and converting the imagery into aform suitable for processing by a computer without sacrifice ofresolution. A second object is to provide the high-resolution scanningcapability without requiring exceedingly-precise and costly transportmechanisms. A third object is to provide the capability of scanningcolor media with high resolution without the necessity of using acolor-corrected lens.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an oblique projection of the high-resolution scanningapparatus.

FIG. 2 is a sectional view of the high-resolution scanning apparatustaken upon the plane 2--2 of FIG. 1.

FIG. 3 is a sectional view of the high-resolution scanning apparatustaken upon the plane 3--3 of FIG. 1.

FIG. 4 is an oblique projection of the optical assembly.

FIG. 5 is a sectional view of the optical assembly taken upon the plane5--5 of FIG. 4.

FIG. 6 is an oblique projection of the path-folding 45-degree mirrorsmounted on the end plates.

FIG. 7 is a sectional view of the path-folding 45-degree mirrors mountedon the endplates taken upon the plane 7--7 of FIG. 7.

FIG. 8 is a block diagram of the z-interferometer processor.

FIG. 9 shows the waveforms that illustrate the operation of thez-interferometer processor.

FIG. 10 is a sectional view of the x-axis interferometer taken upon theplane 10--10 of FIG. 4.

FIG. 11 is a block diagram of the processing electronics for thehigh-resolution scanning apparatus.

FIG. 12 is the flow diagram that shows the operations performed by themicroprocessor.

FIG. 13 is the flow diagram for the detector compensation routine.

FIG. 14 is the flow diagram for the TA interrupt routine. FIG. 15 is theflow diagram for the SA interrupt routine.

FIG. 16 shows regions of the media support panel that are used incalibrating the light detectors and in determining the values ofconstants in the equations that relate the coordinates of the pixelsimaged on the light detectors to the coordinates of a reference pixel.

FIG. 17 is the flow diagram for the detector coordinate compensationroutine.

FIG. 18 is the flow diagram for the TB interrupt routine.

FIG. 19 is the flow diagram for the SB interrupt routine.

FIG. 20 is the flow diagram for the calculate routine.

FIG. 21 is the flow diagram for the TC interrupt routine.

FIG. 22 is the flow diagram for the SC interrupt routine.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An overall view of the preferred embodiment of the high-resolutionscanning apparatus is shown in FIG. 1. The mechanical, electrical, andelectronic portions of the apparatus are contained within the enclosure102 except for a portion of the scanning mechanism 106 that extendsthrough the slots 110 in the top surface of the enclosure and isconcealed by the support frame cover 114. The media support assembly118, 122 in the top surface of the enclosure 102 consists of the glasspressure panel 118 on top of the glass media support panel 122. Thepressure panel can be raised to permit image-containing media to besandwiched between the two panels during the scanning process, Duringthe scanning process, the slots 110 permit the portion of the scanningmechanism concealed beneath the support frame cover 114 to movelengthwise and access all portions of the image-containing media. Thecontrol panel 124 permits a user to exercise control over the operationof the scanning apparatus.

Sectional views of the scanning apparatus taken upon the planes 2-2 and3-3 are shown in FIGS. 2 and 3 respectively. The media support panel 122forms part of the top surface of the enclosure 102. The pressure panel118 is hinged to the top surface of the enclosure 102 at the edge 126 sothat it can be raised for the purpose of inserting an image-containingmedium between the pressure panel 118 and the support panel 122. Thepurpose of the pressure panel is to hold the medium in intimate contactwith the support panel during the scanning process.

Scanning of the medium in two dimensions is accomplished by the y-axisguide-rail transport mechanism 130, the lower x-axis guide-railtransport mechanism 134, and the upper x-axis guide-rail transportmechanism 138. The y-axis transport mechanism 130 consists of the y-axisdouble-rail assembly 142 which is mounted on the structural member 146that is fastened to the enclosure 102 and the y-axis bearing blockassembly 150. The y-axis double-rail assembly 142 includes two hardenedand ground steel rails which make contact with the y-axis bearing blockassembly 150 via steel ball bearings contained in cages within they-axis bearing block assembly. A guide-rail transport, mechanism ofsufficient accuracy--less than 10 micrometers per meter of length--isavailable from Techno, a division of Designatronics, Inc, and describedin Techno's Linear Motion Products Catalog (TI-832) (copyright 1989).

The y-axis bearing block assembly 150 is driven along the y-axisdouble-rail assembly 142 by means of the lead screw 154 havinganti-backlash characteristics achieved by utilizing a preloaded nutassembly in t,he y-axis bearing block assembly 150. The lead screw 154is driven by the reversible y-axis motor 158.

The lower x-axis transport, mechanism 134 consists of the x-axisdouble-rail assembly 162 and the x-axis bearing block assembly 166. Thelower x-axis transport, mechanism 134 is mounted within the supportframe 170 which in turn is mounted on the y-axis bearing block assembly150 of the y-axis transport, mechanism 130. The optical assembly 174 ismounted on the tilting platform 178 which in turn is mounted on thex-axis bearing block assembly 166 of the lower x-axis transportmechanism 134. The support frame 170 extends through the slots 110 (seeFIG. 1) thereby providing a means for supporting portions of thescanning apparatus above the media support assembly 118, 122.

The upper x-axis transport, mechanism 138 consists of the x-axisdouble-rail assembly 182 which is mounted to the support frame 170 andthe x-axis bearing block assembly 186. The support frame cover 114 isnot, shown in FIG. 3. The transmissive media illuminator 190--which doesnot appear in the view shown in FIG. 2 but does appear in FIG. 3--ismounted on the x-axis bearing block assembly 186 of the upper x-axistransport mechanism 138. The transmissive media illuminator 190 can beeither a halogen lamp or three light-emitting diodes having differentcolors.

The x-axis transport mechanisms 134, 138 are similar to the y-axistransport mechanism 130 but may be of lighter construction since theweight they support are significantly less than the weight supported bythe y-axis transport mechanism.

The function of the optical assembly 174 is to image y-axis columnsegments of picture elements ("pixels") of the illuminatedimage-carrying medium on linear arrays of light detectors. The x and ycoordinates of a "reference" pixel are constantly measured by means ofcoherent light beams propagated between the optical assembly 174 andlight-reflecting structurally-rigid position reference members 194 and198 that are attached to two adjacent sides of the enclosure 102. Thereference pixel is the pixel that is imaged on an arbitrarily-selectedreference light detector.

The y-position reference member 194 is attached to the enclosure 102 insuch a way that the light-reflecting surface lies in an x-z plane wherethe z axis is an axis orthogonal to the x and y axes established by thex-axis and y-axis guide rails described above. The y-axis referencemember 198 does not appear in the view shown in FIG. 3.

The x-axis reference member 198 is attached to the enclosure 102 in sucha way that the light-reflecting surface lies in a y-z plane. The x-axisreference member appears in the view shown in FIG. 3 but does not appearin the view shown in FIG. 2.

It is desirable that the pixels lying on an x-axis row of theimage-containing medium are imaged successively on the same detector ina detector array as the x-axis transport mechanism scans across theimage-containing medium. To ensure that this relationship between rowsof pixels and detectors prevails, the optical assembly 174 is tiltedslightly from side to side as the x-axis scan takes place in accordancewith the measured distance between the reference pixel and they-distance reference member 194 thereby maintaining a constant distancebetween the reference pixel (and all other imaged pixels) and they-distance reference member.

The tilting of the optical assembly 174 is accomplished by the tiltingplatform 178 which consists of the U-shaped structural member 202 andthe y piezoelectric device 206 in the "U" opening. Variations in thevoltage applied to the y piezoelectric device 206 cause the distancebetween the legs of the tilting platform 178 to vary thereby causing theoptical assembly 174 to tilt one way or the other.

Variations in the y distance of 5 to 10 micrometers may occur as aresult of manufacturing tolerances associated with the lower x-axistransport mechanism 134. Consequently, the range of adjustment providedby the tilting platform 178 should be at least 10 micrometers.

If the variations in the y distance of the optical assembly 174 as thex-axis transport mechanism scans across the image-containing medium arereasonably consistent from one scan to the next, the x and y coordinatesof a "reference" pixel may alternatively be measured by means ofoptically-readable position-indicating scales that are parallel to thex- and y-coordinate axes and reading devices that are carried by thelower x-axis guide-rail transport mechanism 134.

For example, the y-axis scale could be a thin strip of material attachedto the top surface of the transparent media support panel 122 (FIGS. 2and 3) and running parallel to the edge 842 (FIG. 16) in the detectorarray alignment region 826 (FIG. 16). The y values could be incorporatedin the strip as coded transverse bars, each bar consisting of two setsof 18 segments each. Each segment of the first set would be eitherreflective or absorptive, the particular sequence of reflective andabsorptive segments corresponding to the binary code for its y value.Each segment of the second set would be either reflective ortransparent, the particular sequence of reflective and transparentsegments corresponding to the same binary code as the first set. The ycoordinate of the optical assembly 174 (FIGS. 2 and 3) could bedetermined by reading out either the first or second set of segmentsthat are successively imaged on a particular detector as the opticalassembly begins its x scan. The first set of segments would be utilizedwhen the image-containing medium is being illuminated by the reflectivemedia illuminator 574 (FIG. 3). The second set of segments would beutilized when the image-containing medium is being illuminated by thetransmissive media illuminator 190 (FIG. 3).

The x-axis scale would be binary coded in the same way as the y-axisscale and mounted on the support frame 170 (FIG. 3 parallel to the xaxis, beneath and close to the transparent media support panel 122 (FIG.3), and within the field of view of the optical system that is part ofthe optical assembly 174 (FIG. 3). Since the x-axis scale would bemounted below the image-containing medium, the image-containing mediumobject distance and the x-axis scale object distance would be differentas would the corresponding image distances. Consequently, instead ofutilizing the detectors associated with reading out the pixels of theimage-containing medium, a separate 18-detector linear array mounted atthe appropriate image distance would have to be provided to read out thebinary x data on the x-axis scale.

The x-axis bearing block assemblies 166 and 186 are respectively drivenback and forth along the x-axis double-rail assemblies 162 and 182 bythe belt and pulley system shown in FIG. 3. The bearing block belt 210is a flexible steel belt constrained to follow the path defined byprecision pulleys 214, 218, 222, 226, 230, 234, 238, 242, and 246. Thecantilevered pulley 238 applies a force normal to the bearing block belt210 thereby maintaining the belt under a constant tension. The bearingblock belt 210 is driven by the reversible motor 250, the powertransmission belt 254, and the driving pulley 258 which is affixed topulley 222. Slippage between the bearing block belt 210 and the pulley222 as a result of the power transferred to the pulley 222 from thedriving pulley 258 is prevented by the large frictional force betweenthe bearing block belt 210 and the pulley 222 that results from theapproximate 200 degree angular engagement of the belt and the pulley.

The portion of the bearing block belt 210 extending between the pulleys214 and 218 is attached to the x-axis bearing block assembly 166 and theportion extending between the pulleys 230 and 234 is attached to thex-axis bearing block assembly 186. The portion of the bearing block belt210 that extends between the pulleys 222 and 226 moves freely through anaperture in the bearing block assembly 166.

It should be noted that the arrangement of the bearing block belt 210 issuch that the x-axis bearing block assemblies 166 and 186 travel in thesame direction. The attachment of the bearing block assemblies 166 and186 to the bearing block belt 210 is accomplished in such a manner thatthe center of the transmissive media illuminator 190 falls on theoptical axis of the optical assembly 174. Thus, the pixels of theimage-containing medium that are illuminated by the illuminator 190 arethe ones that are imaged by the optical assembly on the detector arrays.Since the two x-axis bearing block assemblies 166 and 186 move togetheralong their respective x-axis double-rail assemblies 162 and 182, eachpixel along a row of the image-containing medium receives the sameillumination while it is imaged on a particular detector.

The electronics package 262 converts input electrical power to suitablevoltage levels for powering the electrical apparatus and electroniccircuits and performs the control and signal processing functionsnecessary for the operation of the high-resolution scanning apparatus.

An oblique projection of the optical assembly 174, the U-shapedstructural member 202, and the y piezoelectric device 206 is shown inFIG. 4 and a sectional view taken upon the plane 5-5 is shown in FIG. 5.The x-axis interferometer 266, the y-axis interferometer 270, and thez-axis interferometer 274 provide distance measurements to thex-distance reference member 198 (FIG. 3), the y-distance referencemember 194 (FIG. 2), and the media support panel 122 (FIGS. 2 and 3)respectively.

The x-axis interferometer 266 and the y-axis interferometer 270 aremounted near the object pixels on the medium supported within the mediasupport assembly 118, 122 (see FIG. 3). Thus, measurements of thex-distance and the y-distance by the two interferometers are essentiallymeasurements of the x and y coordinates of the object pixels.

The pixel imaging optical system is comprised of the 45 -degree mirror278, the color-corrected, diffraction-limited lens 282, the path-foldingmirrors (not shown in FIG. 4) mounted on end plates 286 and 290, and adetector array assembly located behind the upper left corner of the endplate 286. Light from the illuminated pixels travels downward throughthe apertures in the interferometers 270 and 266, is reflected by mirror278 into lens 282, and then travels to the detector array assembly overa path folded into a compact volume by means of the path-foldingmirrors.

The path-folding 45-degree mirrors 294, 298, 302, 306, 310, 314, 318,and 322 are shown mounted on the end plates 286 and 290 in FIG. 6 and aview taken upon the plane 7-7 is shown in FIG. 7. The light emergingfrom the lens 282 (FIGS. 4 and 5) is reflected from one mirror to thenext, in the order listed above, as illustrated by the central light ray326, and after the last reflection is imaged on the detector array 330.

The high-resolution scanning apparatus is intended forapplications-requiring the resolution of pixels spaced less than 2.5 μmapart. To resolve two red pixels spaced say 2 μm apart the anglesubtended by the lens at the pixel must be approximately 23 degrees. Fora lens having a diameter of 5 cm the object distance, the distance frompixel to lens, should be no greater than about 12.6 cm. Since thedetectors in a linear array are typically spaced approximately 12 μmapart, the optical system should have a magnification of 12 μm dividedby 2 μm or 6 and the image distance, the distance from lens to detector,must be approximately 75.8 cm. The object and image distances translateinto a focal length of approximately 10.8 cm and an f-number for thelens of 10.8 cm divided by 5 cm or 2.2.

The detector array 330 is comprised of three separate linear arrays,each array being comprised of approximately 5000 photodiodes. A redbandpass filter is superimposed on one of the arrays, a blue bandpassfilter on a second, and a green bandpass filter on the third, Thus, thefirst array is sensitive only to red light, the second only to bluelight, and the third only to green light.

By using light-emitting diodes (LEDs) having different colors as themedia illuminators 190, 574 (FIG. 3) and arranging for each LED toilluminate sets of pixels that are imaged on different light detectorarrays, the use of color filters on the detector arrays can be avoided.

A narrowband rejection filter tuned to the wavelength of light used bythe interferometers is placed immediately in front of all three detectorarrays which prevents the light from the interferometers from reachingthe detectors without substantially reducing the pixel light thatarrives at the detectors.

Each of the detector arrays is associated with a charge-coupled-device(CCD) analog shift register whereby the charges accumulated in thedetector potential wells are periodically transferred to the adjoiningpotential wells of the CCD shift register by application of a transferpulse. The charges residing in the shift register after transfer arethen shifted along the shift register and outputted as voltage levelswhen they reach the end. While the shift register contents are beingshifted out, light-generated charge is accumulated in the detectorpotential wells in anticipation of the next transfer pulse which occursafter the shift register has been emptied of charge.

A suitable detector array is the Kodak KLI-5603 detector arraymanufactured by the Microelectronics Technology Division of the EastmanKodak Company. The KLI-5603 detector array is a high-resolutionmultispectral array consisting of three separate linear photodiodearrays with 5632 active photosites for the output of red, green, andblue signals.

The three linear arrays in detector array 330 are aligned with columnsof pixels, a column of pixels being parallel to the y-axis, andconsequently segments of pixel columns of the image-containing mediumare imaged on the linear arrays. During any particular detectorintegration period, one linear array is being exposed to red lightemanating from pixels on one column, another linear array is beingexposed to green light from pixels on another column, and the thirdlinear array is being exposed to blue light from pixels on a thirdcolumn. As the optical assembly 174 is transported along the x-axisdouble-rail assembly 162 (see FIG. 3), the red, green, and bluecomponents of the pixel light for some 5000 rows of the image-containingmedium are measured sequentially by column.

An alternative to the diffraction-limited lens that has minimalaberrations for all visible wavelengths is a less expensive lens that isdiffraction limited for red, blue, or green light but whose focal lengthis a function of wavelength. To utilize such a lens in thehigh-resolution scanning apparatus, the three linear arrays that make upthe detector array 330 would form a stair step configuration such thatthe image distance to each linear array would be in keeping with theconstant object distance and the focal length of the lens for the lightcolor to which the linear array is sensitive.

The detector array assembly 334 includes the detector array 330, thepivoting light shield 338, and the light shield actuator 342. Thepivoting light shield 338 remains in the position shown in FIG. 6 exceptwhen detector dark current is being measured. When dark currentmeasurements are to be made, a solenoid in the shield actuator 342 isenergized causing the light shield 338 to pivot about the light shieldedge 346 and come into intimate contact with the detector array 330. Thepart of the light shield 338 that interfaces with the detector array 330is made of a pliable opaque material that conforms with the surface ofthe detector array and thereby shields the detector array from anylight.

The detector array assembly 334 is mounted on the z piezoelectric device350 which in turn is attached to the end plate 286. The application of avoltage to the z piezoelectric device 350 causes the device to expand byas much as 10 micrometers thereby shifting the detector array assembly334 away from the end plate 286 and shortening the distance between thedetector array 330 and the lens 282. The distance between the detectorarray 330 and the lens 282 is adjusted automatically by means of acontrol circuit and the z piezoelectric device 350 to compensate for anyvariations in the distance between the lens 282 and the media supportpanel 122 that may occur during the scanning process thereby avoidingdefocusing of the pixel images at the detectors. The aforementionedcontrol circuit generates the voltage applied to the z piezoelectricdevice 350 on the basis of the distance to the media support panel 122measured by the z-interferometer 274.

The design of the z-interferometer 274 is shown in FIG. 5. Narrow-bandlight from the laser diode 354 is converted to a plane wave by the lens358. The plane wave is split into two equal components by beamsplitter362, the transmitted reference component 366 and the reflected probecomponent 378. The reference component 366 is reflected by the mirror374 and returns to the beamsplitter 362 where it is split into atransmitted component (which can be ignored) and a reflected component378. The probe component 370 travels to the media support assembly 118,122 (FIGS. 2 and 3) through the aperture 382 in the 45-degree mirror278, the aperture 386 in the support member 390, the aperture 394 in thex-interferometer 266, and the aperture 398 in the y-interferometer 270.The media support assembly 118, 122 splits the probe component 370 intoa transmitted component (which can be ignored) and a reflected component402. The reflected component 402 returns to the beamsplitter 362 whereit is split into a reflected component (which can be ignored) and atransmitted component 406. The reference component 378 combines with theprobe component 406 and the combination plane wave is focused by thelens 410 on the photodetector 414.

The mirror 374 is mounted on the piezoelectric device 418 which in turnis attached to the z-interferometer housing 422. A square-wave voltagesignal applied to the piezoelectric device 418 causes the device toexpand and contract thereby causing the mirror 374 to move in and out(in a direction normal to the mirror surface) by one-eighth wavelengthof the light at the square-wave frequency. As a result, the length ofthe path traveled by the reference component 378 beginning at the laserdiode 354 and ending at the photodetector 414 varies by one-quarterwavelength at the square-wave frequency. If the length of the pathtraveled by the probe component 406 remains fixed, the relative phase ofthe reference component 378 and the probe component 406 varies byone-quarter wavelength at the square-wave frequency.

The photodetector 414 provides a measure of the phase difference betweenthe two components by producing an electrical signal that ranges betweena maximum when the two components are in phase to a minimum when the twocomponents are out of phase. A narrowband transmission filter centeredon the wavelength of the interferometer light is placed over thephotodetector 414 so as to minimize the effects of any pixel light thatarrives at the photodetector.

The reference component 378 travels a path from the laser diode 354 tothe photodetector 414 that remains fixed in length except for thevariations introduced by the piezoelectric device 418. The probecomponent 406 travels a path between the same beginning and end pointsthat may vary slightly in length because of mechanical imperfections inthe x-axis and y-axis transport mechanisms 134, 130. Thus, in theabsence of the phase modulation introduced by the piezoelectric device418, the two components 378 and 406 will move in and out of phase as thedistance to the media support assembly 118, 120 increases or decreases.The change in the distance between the z-interferometer 274 and themedia support assembly 118, 120 can be monitored by accumulating withregard to sign the half-cycle changes in phase as the two components 378and 406 move in and out of phase.

The accumulation of half-cycle changes in phase is accomplished by thez-interferometer processor 424 shown in FIG. 8. The output signal fromthe photodetector 414 when the distance between the z-interferometer 274and the media support assembly 118, 120 (FIGS. 2 and 3) is increasingsteadily is the square wave 426 shown in FIG. 9. The odd-numbered levels430 and the even-numbered levels 434 of the square wave 426, the levelsbeing numbered from left to right beginning with one, are bounded by asine wave and a cosine wave respectively. The shifting back and forthbetween odd and even levels is produced by the phase modulation of thereference component 378 (FIG. 5) that results from applying thesquare-wave voltage signal to the z piezoelectric device 350.

The odd levels 430 and the even levels 434 (FIG. 9) are gated throughSPST switches 438 and 442 respectively (FIG. 8), the two switches beingcontrolled by the piezoelectric device modulating signal and itsinverse. The envelopes of the odd levels 430 and the even levels 434(FIG. 9) are then obtained by envelope detectors 446 and 450respectively (FIG. 8). The signals from the envelope detectors arepassed through infinite clippers 454 and 458 which convert the biasedinput signals into square waves 462A and 466A (FIG. 9) while maintainingthe same bias crossings. The bias levels are set roughly half waybetween the anticipated maximum and minimum values of the photodetectoroutput signal.

To better illustrate the process, it is assumed for the lower groups ofwaveforms in FIG. 9 that the distance to the media support assembly 118,122 (FIGS. 2 and 3) first increases and then decreases. The first groupof waveforms are identified by numbers to which the letter A has beenappended. The second group of waveforms are identified by numbers towhich the letter B has been appended. A number designating an outputsignal in FIG. 8 stands for either, The square wave 462A has experienceda bias crossing just before the distance starts to decrease and areverse bias crossing τ seconds late. This particular situation ischosen for illustration in that it presents a challenge to the signalprocessing circuitry insofar as maintaining an accurate half-cycle countof the change in phase as the distance increases and decreases overtime.

The square wave 462A is added to a τ-delayed version of itself by themodulo 2 adder 470 with the resulting pulse train 474A shown in FIG. 9.Each pulse of the pulse train 474A corresponds to a bias crossing of theoutput signal from the envelope detector 446 (FIG. 8) and represents anaccumulated change in phase between the two components 378 and 406 (FIG.5) of one-half cycle. It should be noted that two bias crossings areindicated by the two transitions occurring in close succession near thecenter of square wave 462A (FIG. 9). The interval between transitionswas so short, however, that the square wave 474A failed to record it.Thus, if the pulses of pulse train 474A were counted, the accumulatedcount would not be an accurate measure of the change in phase that hadoccurred. The square wave 462A indicates that from one end to the otherthere has been no change in phase. However, pulse train 474A suggeststhat there have been two positive half-cycle changes in phase and onenegative half-cycle change for a net positive half-cycle change inphase.

Any circuit for counting transitions in a square wave will fail to counta transition that follows a prior transition within a time interval τwhere τ depends on the response times of the semiconductor devices usedin the counting circuit. Thus, there is a risk of an inaccuratetransition count unless some means is provided for compensating formissed transitions.

The solution to the problem of not detecting a bias crossing that occursshortly after a prior bias crossing is to gate the square wave 474A insuch a way as to eliminate a pulse that records the occurrence of a biascrossing that is closely followed by a reverse bias crossing. Theaccumulated count of the pulses in the gated pulse train then providesan accurate measure of the phase change except for the very briefintervals when a bias crossing is closely followed by a reverse biascrossing.

The gating signal is obtained in the following way. First, the squarewave 462A is added to the square wave 466A by the modulo 2 adder 478thereby generating the square wave 482A. Then the square wave 482A isdelayed by 3τ to obtain square wave 486A. Finally, the addition ofsquare waves 482A and 486A by modulo 2 adder 490 (FIG. 8) results in thepulse train 494A (FIG. 9).

The pulse train 498A, obtained by delaying pulse train 474A by τ, isAND'ed with pulse train 494A (see FIG. 8) to give the pulse train 502A,the pulses of which are accumulated in the up/down counter 506 (FIG. 8).It should be noted that the middle pulse of pulse train 474A, whichrecorded a bias crossing that was followed shortly thereafter by areverse bias crossing, is eliminated in the pulse train 502A.

The direction of counting for up/down counter 506 (FIG. 8) is controlledby the square wave 510A which is obtained by delaying the square wave482A by τ. If the square wave 510A is high just before a pulse of pulsetrain 502A, the up/down counter 506 counts up. If the square wave 510Ais low, the up/down counter 506 counts down. Thus, the square wave 510Acauses the up/down counter 506 to count up when the first pulse of pulsetrain 502A occurs and down when the second pulse occurs, the change incount after the two pulses have occurred being zero, as it should be.

A slightly different situation is illustrated in FIG. 9 beginning withsquare waves 462B and 466B. The bias crossing that occurs while thedistance is increasing is followed after a 2τ interval by a reverse biascrossing that occurs while the distance is decreasing. The waveforms474B through 510B follow from square waves 462B and 466B as before.

The bias crossing and the reverse bias crossing that follows after a 2τtime interval are spaced sufficiently far apart in time to be recordedas a pair of pulses near the center of the pulse train 502 B. Sincesquare wave 510B is high just before the first pulse of the pair occurs,the first pulse results in the up/down counter 506 being incremented.Since the square wave 510B is low just before the second pulse of thepair occurs, the second pulse results in the up/down counter 506 beingdecremented. Thus, the count registered by the up/down counter after thefour pulses of pulse train 502B have been counted remains the same as itwas before.

There are many possible ways of preventing the counting of either of twoclosely-spaced transitions which are equivalent in function to thecircuit shown in FIG. 8. These other ways should be considered to bealternative embodiments of the transition-counting circuitry of thepresent invention.

The contents of the up/down counter 506 are clocked into the register514 periodically by the register clock signal 518 which has a frequencyof at least 10 kHz.

The x-axis interferometer 266 (FIG. 4) and the y-axis interferometer 270(FIG. 4) are essentially identical. A sectional view of the x-axisinterferometer 266 taken upon the plane 10-10 is shown in FIG. 10.Monochromatic light that emerges from the laser diode 522 is collimatedby the lens 526 and the resulting plane wave 530 is split into twocomponents--the reflected reference wave 534 and the transmitted probewave 538--by the beamsplitter 542.

The reference wave 534 travels to the mirror 546 and returns as the wave550 to the beamsplitter 542 where it is split into the transmitted wave554 and the reflected wave which can be ignored.

The probe wave 538 travels to the x-distance reference member 198 (FIG.3), is reflected, and returns as the wave 558 to the beamsplitter 542where it is split into a reflected wave 562 and a transmitted wave whichis can be ignored. The reference wave 554 and the probe wave 562 combinein a phase relationship that depends on the difference in distancestraveled by the two waves since leaving the laser diode 522. Thecombined wave consisting of the reference wave 554 and the probe wave562 is focused by lens 566 on the photodetector 570 which produces anelectrical signal that is a maximum when the two waves 554 and 562 arein phase and a minimum when the two waves are out of phase.

The change in distance between the x-axis interferometer 266 and thex-distance reference member 198 during scanning can be determined bycounting the occasions when the photodetector output signal goes fromhigh to low and from low to high. Each time the photodetector outputsignal makes the high-to-low or low-to-high transition represents achange in distance of one-quarter wavelength of the light beam. Thecounting of the half-cycle phase changes between the two waves 554 and562 is accomplished by circuitry similar to that shown in FIG. 8.

The operation of the y-axis interferometer 270 is similar to the x-axisinterferometer 266 except that it involves reflections of the light wavefrom the y-distance reference member 194. For both the x-axisinterferometer and the y-axis interferometer, the register clock signal518 (FIG. 8) is the signal that causes the transfer of charge betweenthe detector array and the CCD analog shift register. Thus, each timethe detector array is read out, corresponding values for the x and ydistances are available.

Mounted in the top of the y-axis interferometer 270 (FIG. 3) close tothe pixels that are imaged on the detector array is the reflective mediailluminator 574. The reflective media illuminator 574 can be either ahalogen lamp or three light-emitting diodes having different colors.

The processing electronics for the high-resolution scanning apparatusshown in FIG. 11 is contained in the electronics package 262 (FIG. 3).The operation of the scanning apparatus is controlled by themicroprocessor 578 by means of the control bus 582 and the data bus 586.A word placed on the control bus 582 by the microprocessor 578 operatesas a command to a particular unit to perform a particular function suchas placing data on the data bus 586 for the microprocessor to read orreading data that was placed on the data bus by the microprocessor. Themicroprocessor 578 is alerted to an immediate processing need byinterrupt signals (not shown in the figure) from the control panel 124and the timing generator 590. Commercially available microprocessorsthat can perform the functions of the microprocessor 578 are Intel's80386 and Motorola's 60030.

The user of the scanning apparatus interfaces with the microprocessor578 by means of the control panel 124. The control panel 124 providesthe means for turning power on, for selecting either the transmissivemedia or reflective media mode of operation, for selecting the calibratemode or the operate mode of operation, and for selecting a particularrectangular region to be scanned. Alternatively, some or all of thesecontrol functions could be exercised by the user of the apparatus bymeans of an external computer.

All timing signals necessary for the operation of the apparatus aregenerated by the timing generator 590. The interconnection of the timinggenerator 590 and the units which utilize timing signals generated bythe timing generator are not shown in the figure.

The reflective media illuminator switch 594 and the transmissive mediailluminator switch 598 turn the reflective media illuminator 574 (FIG.3) and the transmissive media illuminator 190 (FIG. 3) respectively onor off and are under the control of the microprocessor 578.

The three CCD analog shift registers associated with the three lineardetector arrays that comprise the detector array 330 output throughthree ports that are sequentially and periodically connected by means ofthe single-pole triple-throw switch 602 to the analog-to-digital (A/D)converter 606. The switching of switch 602 is automatically accomplishedby a timing signal supplied by timing generator 590 at a rate threetimes higher than the transfer rate from each linear detector array toits associated CCD shift register.

The analog CCD shift register outputs are converted to digitalrepresentations by the A/D converter 606. The results of the A/Dconversion process are read by the microprocessor 578 as they becomeavailable.

The x-axis interferometer processor 610 keeps track of the round-tripdistance between the x-axis interferometer 266 and the x-distancereference member 198 in half-cycle increments. This distance can beaccessed by the microprocessor 578 and is also made available to thex-axis motor controller 614.

The x-axis motor 250 is caused to drive the optical assembly 174 eitherto within a predetermined range of a desired x-distance or at a desiredx-velocity by the microprocessor 578 writing the desired x-distance orthe desired x-velocity into a register in the x-axis motor controller614. In the positioning mode of operation, the x-axis motor controller614 supplies current via the digital-to-analog (D/A) converter 618 tothe x-axis motor 250 based on (1) the difference between the desiredx-distance and the measured x-distance provided by the x-axisinterferometer processor 610 and (2) the rate of change of the measuredx-distance. In the velocity-maintenance mode of operation, the x-axismotor controller 614 supplies current to the x-axis motor 250 based onthe rate of change of the measured x-distance alone.

The x-axis park microswitch 622 is activated whenever the opticalassembly 174 reaches the x-axis "park" position--the far left positionin FIG. 3 where the ends of the x-axis bearing block assembly 186 andthe x-axis double rail assembly 182 are aligned. The activation of thex-axis park microswitch 622 causes the x-motor controller to stop thex-axis motor 250.

The y-axis interferometer processor 626 keeps track of the round-tripdistance between the y-axis interferometer 270 and the y-distancereference member 194 in half-cycle increments. This distance can beaccessed by the microprocessor 578 and is also made available to they-axis motor controller 630 and the y-axis piezoelectric devicecontroller 634.

The y-axis motor 250 is caused to drive the support frame 170 (FIG. 2)to within a predetermined range of the desired y-distance by themicroprocessor 578 writing the desired y-distance into a register in they-axis motor controller 630. When the motion of the support frame 170ceases, the zero rate of change of the measured y-distance alerts they-axis motor controller 630 to turn off the power to the y-axis motor158 (FIG. 2) and to activate the y-axis piezoelectric device controller634.

The y-axis piezoelectric device controller 634 then applies a voltagebased on the difference between the measured and desired y-distances tothe y-axis piezoelectric device 206 (FIG. 4) via the digital-to-analog(D/A) converter 638 causing the piezoelectric device to expand orcontract thereby tilting the optical assembly 174 until a position isreached where the measured and the desired y-distances are equal.

The equality of the measured and the desired y-distances are maintainedby the y-axis piezoelectric device controller 634 as the opticalassembly 174 (FIG. 3) is moved parallel to the x-axis by the lowerx-axis transport mechanism 134.

The y-axis park microswitch 640 is activated whenever the support frame170 (FIG. 2) reaches the y-axis "park" position--the far right positionin FIG. 2 just before the optical assembly 174 makes contact with theend wall of the enclosure 102. The activation of the y-axis parkmicroswitch 640 causes the y-axis motor controller 630 to stop they-axis motor 158.

As the optical assembly 174 (FIGS. 2 and 3) scans the image-containingmedium, the object distance (i.e. the distance from the object pixels tothe lens 282) may vary to such a degree as to exceed the depth of focusof the lens. In order to compensate for any variations in the objectdistance, the image distance (i.e. the distance between the detectorarray 330 (FIG. 5) and the lens 282 (FIG. 5)) is adjusted by means ofthe z-axis piezoelectric device 350 (FIG. 5).

A table of values relating the z-distance measured by the z-axisinterferometer (which is linearly related to the object distance) andthe voltage to be applied to the z-axis piezoelectric device 350 toachieve the appropriate image distance are stored in the z-axispiezoelectric device controller 642, During scanning, the z-axispiezoelectric device controller 642 translates the z-distance suppliedby the z-axis interferometer 274 to a voltage which it supplies via thedigital-to-analog (D/A) converter 646 to the z-axis piezoelectric device350. The z-axis piezoelectric device 350 expands or contracts as theobject distance varies thereby keeping the object pixels imaged on theinfrared light detectors.

For each pixel scanned the microprocessor 578 outputs the x and ycoordinates of the pixel and pixel light intensity G as a function oflight color c. The data is placed in x holding register 650, y holdingregister 654, G holding register 658, and c holding register 662 as itbecomes available. The data held in the holding registers 650, 654, 658,and 662 are transferred by the transfer clock signal 682 from the timinggenerator 590 to the x, y, G, and c output registers 666, 670, 674, and678 just prior to the availability of new data. The aforementionedoutput registers are accessible to an external computational facilitywhich is alerted each time new data becomes available by the transferclock signal 682.

The operations of the microprocessor 578 are governed by the flowdiagrams shown in FIG. 12. When the user presses the "power on" switch,the microprocessor is initialized. When the user presses the "start"switch, the microprocessor is directed to the start 686 of the mainprogram shown at the top left of FIG. 12. The microprocessor firstperforms the test 690 to determine the state of the operate/calibrateswitch on the control panel 124 (FIGS. 1 and 11). If theoperate/calibrate switch is set to "calibrate", the microprocessor goesto the detector compensation routine 694. If the switch is set to"operate", the microprocessor performs the test 698 to determine thestate of the transmissive/reflective media switch on the control panel124 (FIGS. 1 and 11).

If the transmissive/reflective media switch is set to "reflectivemedia", the microprocessor performs operation 702 and writes a commandto the reflective media illuminator switch 594 (FIG. 11) turning thereflective media illuminator 574 (FIGS. 3, 4, and 5) on and enters 1into the p register. If the media switch is set to "transmissive media",the microprocessor performs operation 706 and writes a command to thetransmissive media illuminator switch 598 (FIG. 11) turning thetransmissive media illuminator 190 (FIG. 3) on and enters 2 into the pregister.

The values of p stored in the p register indicate the mode of operationof the scanning apparatus. A p value of 0 indicates that the detectorarray 330 is shielded by the light shield 338 (FIG. 6). A p value of 1indicates that the reflective media illuminator is turned on. A p valueof 2 indicates that the transmissive media illuminator is turned on.

After performing operations 702 or 706 the microprocessor executes the"goto" command 710 to the operate routine 714 shown at the top right ofFIG. 12.

The microprocessor causes the message "enter x_(min) " to be displayed718 on the control panel 124 (FIGS. 1 and 11) which instructs the userto enter the x coordinate of the left edge of the region of theimage-containing medium to be scanned. The microprocessor waits forx_(min) to be entered 722 and then causes the message "enter x_(max) "to be displayed 726 on the control panel, x_(max) being the x coordinateof the right edge of the region to be scanned.

When the user has completed the entry of x_(max) 730, the microprocessorcauses the message "enter y_(min) " to be displayed 734 on the controlpanel which instructs the user to enter the y coordinate of the bottomedge of the region to be scanned.

When the user has completed the entry of y_(min) 738, the microprocessorcauses the message "enter y_(max) " to be displayed 742 on the controlpanel, y_(max) being the y coordinate of the top edge of the region tobe scanned.

When the user has completed the entry of y_(max) 746, the microprocessorwrites y_(min) to the y-axis motor controller 630 (FIG. 11) therebycausing the y-axis controller to drive the y-axis transport mechanism130 (FIG. 2) to the vicinity of coordinate y_(min) 750 whereupon they-axis piezoelectric device controller tilts the optical assembly 174(FIG. 2) until y equals y_(min).

The microprocessor reads y 754 and compares y with y_(min) 758,repeating the sequence of operations until y equals y_(min). Then itperforms the operations 762. It writhes 0 into the s (scan direction)register and the scan velocity v_(s) into the v (scan velocity) registerof the x-axis motor controller 614 (FIG. 11) thereby instructing thecontroller to scan in the direction of increasing x at a constantvelocity v_(s). The microprocessor stores the scan direction 0 in the sregister and enables the TC interrupts.

The scanning of an image-containing medium is accomplished by the TC andSC interrupt routines which will be discussed later. When the final xscan has been completed, a "park" flag is set which tells themicroprocessor that the scanning process has been completed.

The microprocessor waits 766 for the "park" flag to be set and thenperforms the operations 770 returning the optical assembly to its parkposition and turning off the transmissive media illuminator 190 (FIG. 3)or the reflective media illuminator 574 (FIG. 3), whichever is turnedon.

If the operate/calibrate switch is set to "calibrate", the comparisonoperation 690 causes the microprocessor to go to the detectorcompensation routine 694 shown in FIG. 13. The microprocessor performsthe operations 774 consisting of entering 0's into the p and wregisters, activating the detector light shield actuator 342 therebycausing the pivoting light shield 338 to cover the detector array 330(see FIG. 6), and enabling the TA interrupts. The dark current for allof the detectors in the detector array are measured while the detectorarray is shielded from ambient light by the light shield.

The TA interrupt signal is the square-wave signal supplied by the timinggenerator 590 (FIG. 11), the low-to-high transition of which causes thetransfer of the contents of the three linear detector arrays thatcomprise detector array 330 (FIG. 6) to the associated CCD shiftregisters. When the rising transition of the TA interrupt signal occurs,the microprocessor immediately performs the operations 778 of the TAinterrupt routine shown in FIG. 14. The SA interrupts are enabled, theTA interrupts are disabled, 1's are entered into the d and c registers,and a 0 is entered into the p register.

The d parameter identifies the detector whose output is currently beingprocessed. The detectors in each of the three linear arrays thatcomprise the detector array 330 (FIG. 6) are numbered from 1 to D, thenumber 1 being assigned to the detector whose output emerges first fromthe associated CCD analog shift register and D being the number ofdetectors in each linear array.

The c parameter takes on the values 1, 2, and 3 and identifies the lightcolor to which the d detector was exposed.

The SA interrupt signal is a square wave supplied by the timinggenerator 590 (FIG. 11), the rising transition of which coincides withthe availability from the A/D converter 606 (FIG. 11) of a digitizedvalue of the output of detector d that was exposed to the light color c.The first rising transition of the SA interrupt signal after enablementof the SA interrupts corresponds to d=1 and c=1, the second transitioncorresponds to d=1 and c=2, the third corresponds to d=1 and c=3, andthe fourth corresponds to d=2 and c=1. The pattern continues until allof the detector outputs have been digitized by the A/D converter.

The rising transition of the SA interrupt signal causes themicroprocessor to immediately perform the SA interrupt routine shown inFIG. 15 after completion of the TA interrupt routine triggered by theprior TA interrupt signal. The first operations performed 782 are thereading of the output V from the A/D converter 606 (FIG. 11) and thestoring of that value at an address in memory uniquely determined by thevalues of p=0, d, and c. The voltages stored in memory as a result ofthe present process are the detector voltages that arise from the darkcurrents in the detectors since the detectors are shielded from ambientlight when these detector readings are made.

The microprocessor then compares d with D 786 where D is the numberassigned to the last detectors in the three linear detector arrays. If dis not equal to D, the d register is incremented 790 and themicroprocessor returns to the detector compensation routine shown inFIG. 13.

If d is equal to D, then c is compared 794 with 3. If c is not equal to3, a 1 is entered into the d register and the c register is incremented798. If c is equal to 3, output values from the A/D converter have beenobtained for all detectors. The SA interrupts are disabled and a 1 isentered into the w register 802. The microprocessor then returns to thedetector compensation routine shown in FIG. 13.

When the microprocessor determines that w equals one 806, it performsthe operations 810 of entering y_(cal1) into the y_(cal) register,deactivating the light shield actuator 342 thereby causing the lightshield 338 to uncover the detector array 330 (see FIG. 6) and writingy_(cal) to the y-axis motor controller 630 (FIG. 11). The quantityy_(cal1) is the y coordinate to which the support frame 170 (FIG. 2) isinitially driven for calibration.

Regions of the media support panel 122 (FIG. 1) important for thecalibration process are shown in FIG. 16. The rectangular outline 814identifies the perimeter of the media support panel 122 that can beexposed to both reflective and transmissive media illumination and thatcan be imaged on the detector array 330 (FIG. 6) during the scanningprocess. The transmissive media calibration region 818 is a region ofuniform transmissivity. The reflective media calibration region 822 is aregion of zero transmissivity and uniform reflectivity. The detectorarray alignment region 826 is a region of zero transmissivity. Theplacement of reflective or transmissive media is restricted to mediaregion 830.

The y axis 834 is the locus of the images of the d=1, c =1 detector onthe media support panel 122 (FIG. 1) as the support frame 170 (FIG. 2)moves in the y direction and the value of x measured by the x-axisinterferometer 266 (FIG. 3) is held constant at some particular value.Similarly, the x axis 838 is the locus of the images of the samedetector as the optical assembly 174 (FIG. 3) moves in the x directionand the value of y measured by the y-axis interferometer 270 (FIG. 3) isheld constant at some particular value. The y axis will in general makea small angle θ_(i) with the edge 842 of the detector array alignmentregion 826. The line 846 through the images of the c=1 detectors on themedia support panel 122 (FIG. 1) will make a small angle θ_(a) with theedge 842. The values of both θ_(i) and θ_(a) are initially unknown andmust be measured during the calibration process.

The value of y_(cal1) is chosen such that all of the detectors indetector array 330 (FIG. 6) are imaged within the transmissive mediacalibration region 818.

After writing y_(cal) to the y-axis motor controller 630 (FIG. 11), y isread 850 and compared with y_(cal) 854. When y equals y_(cal), theoperations 858 are performed. The transmissive media illuminator 190(FIG. 3) is turned on, a 2 is entered into the p register, and x_(t) isentered into the x_(p) register. The quantity x_(p) is the x coordinatethat the optical assembly 174 (FIG. 3) is driven to for calibration. Thevalue of x_(t) is chosen such that all of the detectors in detectorarray 330 (FIG. 6) are imaged within the transmissive media calibrationregion 818.

The microprocessor writes x_(p) to the x-axis motor controller 614 (FIG.11) 862 and then reads x 866 and compares it with x_(p) 870, repeatingthe sequence of operations until x equals x_(p). When that event occurs,a 0 is entered into the w register and the TA interrupts are enabled874.

The execution of the TA interrupt routine (FIG. 14) followed by theexecution of the SA interrupt routine (FIG. 15) for each detector indetector array 330 (FIG. 6) results in the storage of detector voltagescorresponding to detector exposures to a medium of uniformtransmissivity. This data provides the means for calculating theresponsivities of the individual detectors.

The microprocessor waits for w to equal one 878 and then determineswhether p is equal to one 882. If it is not the microprocessor performsthe operations 886 consisting of turning off the transmissive mediailluminator 190 (FIG. 3), turning on the reflective media illuminator574 (FIG. 3), and entering 1 into the p register and x_(r) into thex_(p) register. The value of x_(r) is chosen such that all of thedetectors in detector array 330 (FIG. 6) are imaged within thereflective media calibration region 822. The routine beginning with theoperation 862 is then repeated.

If the transmissivity of the transmissive media calibration region 818or the reflectivity of the reflective media calibration region 822varies over the respective regions to an unacceptable degree, thedetector compensation routine shown in FIG. 13 should be performed anumber of times for different values of y_(cal1) and either x_(t) orx_(r) or both. The average detector outputs should be stored in theV(p,d,c) memory.

If the comparison 882 reveals that p equals 1, the detector compensationroutine has been exercised for all values of p and the microprocessorthen goes to the detector coordinate compensation routine (DCCR) shownin FIG. 17.

The microprocessor writes x_(a) to the x-axis motor controller 614 (FIG.11) 890. The quantity x_(a) is chosen such that the line x=x_(a) iswithin the reflective media calibration region 822 and the detectorarray alignment region 826 for the entire range of y values. Themicroprocessor then reads x 894 and compares x with x_(a) 898. The twooperations are repeated until x equals x_(a) at which time theoperations 902 are performed. A 0 is written into the s (scan direction)register and v_(s) into the v (scan velocity) register of the x-axismotor controller 630 (FIG. 11) which causes the optical assembly 174(FIG. 3) to scan at a constant velocity v_(s) in the direction ofincreasing x. Zeros are entered into the w and f registers and the TBinterrupts are enabled. The f parameter is a control flag, the purposeof which will become apparent during the discussion of the TB and SBinterrupt routines.

The TB interrupt signal is the same as the TA interrupt signal. It isthe square-wave signal supplied by the timing generator 590 (FIG. 11),the low-to-high transition of which causes the transfer of the contentsof the three linear detector arrays that comprise detector array 330(FIG. 6) to the associated CCD shift registers.

When the rising transition of the TB interrupt signal occurs, themicroprocessor immediately performs the operations 906 of the TBinterrupt routine shown in FIG. 18. The microprocessor reads x andenables the SB interrupts. It then determines whether the f flag hasbeen set 910. If it has not been set (which will be true immediatelyafter the TB interrupts were enabled), 1's are entered into the q, d,and c registers.

The q parameter measures the number of SB interrupts that occur afterthe first occurrence of a detector image crossing the boundary betweenthe region consisting of the opaque reflective media calibration region822 and the detector array alignment region 826 and the transparentmedia region 830 (see FIG. 16).

If it is determined that the f flag has been set 910, the q register isincremented 918 and q is compared with Q 922. A total of Q SB interruptsare permitted to occur before changing the y coordinate, the value of Qbeing chosen sufficiently large that the boundary transition will bedetected by all detectors in the array. If q does not equal Q, themicroprocessor returns to the DCCR (FIG. 17). If q does equal Q, themicroprocessor disables the TB interrupts 926 and then returns to theDCCR.

The SB interrupt signal, just like the SA interrupt signal, is a squarewave supplied by the timing generator 590 (FIG. 11), the risingtransition of which coincides with the availability from the A/Dconverter 606 (FIG. 11) of a digitized value of the output of detector dthat was exposed to the light color c. When the rising transition of theSB interrupt signal occurs, the microprocessor immediately goes to theSB interrupt routine shown in FIG. 19, after completion of the TBinterrupt routine triggered by the prior TB interrupt signal, and readsV 930.

The microprocessor next compares V with a threshold voltage V_(t) 934.The threshold voltage V_(t) is a predetermined voltage that is roughlyhalf way between voltages that result from exposure of the detectors tothe opaque regions 822 and 826 and the voltages that result fromexposure of the detectors to the transparent region 830.

If V is less than V_(t), the microprocessor goes immediately to the test942. Otherwise, it performs the operations 938 of setting the f flag andstoring x in the memory location reserved for the opaque/transparentboundary coordinate x_(o/t) (y,d,c) and then determines whether d isequal to D 942. If d is less than D, the output of another detectorremains to be read. The d register is incremented 946 and themicroprocessor then returns to the DCCR to wait for the next SBinterrupt.

If d equals D, a test of c is made 950. If c is less than 3, the outputsof a linear detector array corresponding to another color remain to beread and operations 954 are performed. A 1 is entered into the dregister and c+1 is entered into the c register. The microprocessor thenreturns to the DCCR.

If c equals 3, q is tested 958. If q is less than Q, there may be somedetector images that have not crossed from the opaque regions 822 and826 to the transparent region 830 of the exposed region of the mediasupport panel 814 (see. FIG. 16) and the detector readout process mustcontinue. Ones are entered into the d and c registers 962 and themicroprocessor returns to the DCCR.

If q equals Q, the boundary between the opaque regions 822 and 826 andthe transparent region 830 of the exposed region of the media supportpanel 814 (see. FIG. 16) has been established for the present y positionof the detector array 330 (FIG. 6). The SB interrupts are disabled and a1 is entered into the w register 966. The microprocessor then returns tothe DCCR for the purpose of adjusting y.

Each time the microprocessor returns to the DCCR after performing the TBand SB interrupt routines, it waits (FIG. 17) for w to equal one 970.When this event occurs, the microprocessor enters y_(cal) +δy into they_(cal) register 974 and determines 978 whether y_(cal) is greater thanor equal to y_(cal2) where y_(cal2) is the largest value of y for whichthe boundary between the opaque regions 822 and 826 and the transparentregion 830 of the exposed region of the media support panel 814 (see.FIG. 16) can be determined. If y_(cal) is less than y_(cal2), themicroprocessor writes y_(cal) into the y-axis motor control let 630(FIG. 11) 982. The microprocessor repeatedly reads y 986 and repeats theDCCR when it determines that y equals y_(cal) 990.

If the microprocessor finds hat y_(cal) equals or exceeds y_(cal2) 978,it returns the optical assembly to its "park" position 994 and goes tothe calculate routine shown in FIG. 20 and performs the calculations998.

The voltage V(p,d,c) produced by a detector d exposed to a light color cfrom a pixel illuminated by media illuminator p is given by the equation

    V(p,d,c)=H(p,d,c)G+V(p=0,d,c)                              (1)

where

    H(p,d,c)=R(p,d,c)G.sub.r/t (p)A(p)Ωφ(p),

G is the reflectivity (p=1) or transmissivity (p=2) of the pixelreferenced to G_(r/t) (p); V(p=0,d,c) is the detector voltage thatresults from the detector dark current when the detector is shieldedfrom ambient light; R(p,d,c) is the responsivity of the detector to thelight incident on the color filter that lies on top of the detector;G_(r/t) is either (1) 0 (p=0), (2) the reflectivity (p=1) of thereflective media calibration region 822 (FIG. 16), or (3) thetransmissivity (p=2) of the transmissive media calibration region 818;A(p) is the pixel area; Ω is the solid angle that defines the pixellight that reaches the detector; and φ(p) is the irradiance (power perunit area) of light from the illuminator.

The quantity G for each pixel is the desired output from thehigh-resolution scanning apparatus. From equation (1),

    G=(V(p,d,c)-V(p=0,d,c))/H(p,d,c)                           (2)

The detector voltage that results from the detector dark current isobtained by shielding the detector array 330 (FIG. 6) from ambient lightwhich corresponds to G_(r/t) being equal to 0 in equation (1).

The quantity H(p,d,c) in equation (2) is obtained by setting G equal to1 in equation (1) which corresponds to the calibration mode discussedabove.

    H(p,d,c)=V(p,d,c)-V(p=0,d,c)                               (3)

The reciprocal of the expression to the left of the equal sign inequation (3) is calculated 1002 by the microprocessor in performing thecalculate routine shown in FIG. 20.

The image of detector d that is exposed to light of color c has an xcoordinate x(d,c) that is given by the equation (see FIG. 16)

    x(d,c)=x+(c-1)δx+(d-1)δyθ.sub.a          (4)

where x is the x coordinate measured by the x-interferometer 266 (FIG.3) and which can be taken to be the x coordinate of the d=1, c=1detector, δx is the spacing between the linear detector arrays that areexposed to different colors, and δy is the spacing between detectors inthe same linear detector array. The origin of the coordinate systemshown in FIG. 16 has the coordinates (x_(o),y_(o)) in the coordinatesystem established by the x-axis and y-axis interferometers.

Let x'(d,c) be the value of x(d,c) when the detector image coincideswith the edge 842 of the detector array alignment region 826 (FIG. 16).From the figure,

    x'(d,c)=x.sub.o +θ.sub.i [(y-y.sub.o)+(d-1)δy] (5)

If the expression for x'(d,c) is substituted for x(d,c) in equation (4),the value of x--denoted by x_(t/o) --is given by the equation

    (x.sub.o/t -x.sub.o)=θ.sub.i (y-y.sub.o)-(c-1)δx-(d-1)δy(θ.sub.a -θ.sub.i)(6)

The quantity x_(o/t) is the reading of the x-interferometer 266 (FIG. 3)when the detector corresponding to the indices d and c coincides withthe edge 842 of the alignment region 826 (FIG. 16).

If follows from equation (6) that

    (x.sub.o/tMEAN -x.sub.o)=θ.sub.i (y.sub.MEAN -y.sub.o)-(c-1)δx-(d-1)δy(θ.sub.a -θ.sub.i)(7)

where the means are taken over all available values of y,

The linear mean square regression curve that best represents the datax_(o/t) in terms of y is obtained by substituting the estimates θ_(ie)and θ_(ae) for θ_(i) and θ_(a) respectively in equation (6). Thequantity θ_(ie) is given by the equation

    θ.sub.ie =(1/3D)Σθ.sub.ie (d,c)          (8)

where the sum extends over all values of d and c and

    θ.sub.ie (d,c)=Σ(x.sub.o/t -x.sub.o/tMEAN)(y-y.sub.MEAN)/Σ(y-y.sub.MEAN).sup.2 (9)

where the sum extends over all values of y. The substitution of θ_(ie)and θ_(ae) for θ_(i) and θ_(a) respectively in equation (7) yields theequation

    δyθ.sub.ae =(1/3(D-1))Σδyθ.sub.ae (d,c)(10)

where the sum extends over all values of c and values of d from 2 to Dand

    δyθ.sub.ae (d,c)=[1/(d-1)][θ.sub.ie (y.sub.MEAN -y.sub.o)-(x.sub.o/tMEAN -x.sub.o)-(c-1)δx]+δy.sub.ie(11)

The microprocessor calculates the mean of x_(o/t) (d,c) 1006 (FIG. 20)by summing the N_(y) available values and then dividing by N_(y) whereN_(y) is equal to 1 plus the integer portion of (y_(cal1)-y_(cal2))/δy_(cal). It calculates the mean of y 1010 by summing theN_(y) available values of y and then dividing by N_(y). It calculatesθ_(ie) (d,c) 1014 in accordance with equation (9). It calculates θ_(ie)1018 in accordance with equation (8). It calculates δyθ_(ae) (d,c) 1022in accordance with equation (11). The quantity y_(o) is equal toy_(cal1). The quantity x_(o) is stored in memory as x_(o/t)(y=y_(cal1),d=1,c=1) (see operation 938, FIG. 19). The microprocessorcalculates δyθ_(ae) 1026 in accordance with equation (10). Thecalibration process ends with the completion of the calculation 1026.

The translation of pixel reflectivity or transmissivity into digitaldata begins when the high-resolution scanning apparatus is in the"operate" mode and the TO interrupts are enabled by the microprocessoras part of operations 762 (FIG. 12). The TC interrupt signal is the sameas the TA and TB interrupt signals. It is the square-wave signalsupplied by the timing generator 590 (FIG. 11), the low-to-hightransition of which causes the transfer of the contents of the threelinear detector arrays that comprise detector array 330 (FIG. 6) to theassociated CCD shift registers.

When the rising transition of the TC interrupt signal occurs, themicroprocessor immediately performs the operations 1030 of the TCinterrupt routine shown in FIG. 21. Ones are entered into the d and cregisters and x is read.

The image of a detector on the image-containing medium constitutes apixel. The pixel is intended to be no larger than a few micrometers insize and can be significantly less, depending on the magnification ofthe optical system. For a detector having an active area of say 12 μm×12μm and an optical system with a magnification of 6, the pixel dimensionsare 2 μm×2 μm.

Since the optical assembly 174 (FIG. 3) is scanning in the x directionat a constant velocity, the interrupt frequency and the scanningvelocity are chosen such that the centroid of the detector active areascans from one edge of a pixel to the opposite edge in the time betweeninterrupts. For a 2-μm pixel and a scanning velocity of 1 mm/s, theinterrupt frequency should by 500 Hz.

The microprocessor next tests the direction of scan s 1034. If s equals1 (scan in direction of decreasing x or from right to left in terms ofFIGS. 1 and 16), the microprocessor determines whether x is less than orequal to x_(max) 1038. If it is not, the microprocessor returns to theoperate routine (FIG. 12). If it is, the microprocessor determineswhether x is less than X_(min) 1042. If it is not, the x-axis transportmechanisms 134,138 are up to speed and causing the desired region (i.e.between x_(min) and x_(max)) of the image-containing medium to bescanned. The microprocessor enables the SO interrupts 1046 and returnsto the operate routine to wait for the interrupts to occur.

If x is less than x_(min), the microprocessor disables the SC interrupts1050 and determines whether x is less than x_(min) -Δx. If it is not,the microprocessor returns to the operate routine. If it is, it is timeto prepare for the reverse scan 1058. The microprocessor enters 0 intothe s register and y_(d) +Dδy into the y_(d) register where D is thenumber of detectors in each of the three linear arrays that comprisedetector array 330 (FIG. 6) and δy is the spacing between the detectorimages on the media support panel 122 (FIG. 2).

Each x-axis scan causes the images of the detectors that make up each ofthe three y-aligned linear detector arrays to sweep across theimage-containing medium. By scanning back and forth in x at intervals ofDδy in y, the pixels in the desired region are read out completely withno overlap.

The use of the tilting platform 178 (FIGS. 2 and 3) can be avoided byscanning back and forth in x at intervals of Mδy where M is some integerless than D. Without the corrections in y provided by the tiltingplatform, the boundaries of the scanned strip may wander back and forthin the y dimension by as much as 5 to 10 rows. However, by choosing Mequal to say D-20, the user is assured of obtaining complete pixel datafor the region. Redundant data can be recognized and eliminated by theappearance of multiple transmissivity/reflectivity values correspondingto the same x and y coordinates.

If the scan direction is in the direction of increasing x (s=0) 1034,the microprocessor compares x with x_(min) 1062. If x is less thanx_(min), the microprocessor returns to the operate routine. If x isequal to or greater than x_(min), the microprocessor compares x withx_(max) 1066. If x is equal to or less than x_(max), the x-axistransport mechanisms 134, 138 are up to speed and causing the desiredregion (i.e. between x_(min) and x_(max)) of the image-containing mediumto be scanned and the microprocessor enables the SC interrupts 1070.

If x is greater than x_(max), the microprocessor disables the SCinterrupts 1074 and compares x with x_(max) +Δx 1078. If x is less thanx_(max) +Δx, the microprocessor returns to the operate routine.Otherwise, the microprocessor prepares for the reverse scan by entering1 into the s register and y_(d) +Dδy into the y_(d) register.

After performing operations 1058 and 1082, the microprocessor compares ywith y_(max) 1086. If y is less than y_(max), the desired portion of theimage-containing medium has not been completely scanned and themicroprocessor writes y_(d) and s into the y-axis motor controller 630(FIG. 11) 1090. This last operation causes the y-axis transportmechanism 130 to move to y=y_(d) and the x-axis transport mechanisms134, 138 (FIG. 3) to begin a scan. Each time the x scan is halted andthe direction of scanning is reversed, the x-axis transport mechanisms134, 138 must travel a distance Δx before reaching the region of theimage-containing medium to be scanned. The quantity Δx is of such amagnitude that the y-axis transport mechanism 130 reaches y_(d) and thex-axis transport mechanisms reach constant velocity before pixel readoutbegins.

If y is equal to or greater than y_(max) 1086, the scanning process hasbeen completed and the microprocessor sets the "park" flag and disablesall interrupts 1094. It then returns to and completes the operateroutine.

If x is greater than or equal to x_(min) and less than or equal tox_(max), the SC interrupts are enabled as a result of the operations1046 and 1070. The SO interrupt signal, just like the SA and SBinterrupt signals, is a square wave supplied by the timing generator 590(FIG. 11), the rising transition of which coincides with theavailability from the A/D converter 606 (FIG. 11) of a digitized valueof the output of detector d that was exposed to the light color c. Whenthe rising transition of the SC interrupt signal occurs, themicroprocessor immediately goes to the SC interrupt routine shown inFIG. 22 and performs the operations 1110.

The microprocessor first reads x and V and then writes into the xholding register 650 (FIG. 11) the expression given by equation (4) withthe δyθ_(ae) calculated during the calculate routine (FIG. 20)substituted for δyθ_(a) and the extra term sδx_(s) added. The quantityδx_(s) is the distance traveled by the optical assembly 174 (FIGS. 2 and3) between TC interrupts. This expression without the sδx_(s) termcorresponds to the x coordinate at the time of the last TC interrupt ofthe image of detector d that is exposed to light of color c. It alsocorresponds to the right-hand edge of the pixel from which most of thelight emanates when the scan is from left to right and to the left-handedge when the scan is from right to left. The sδx_(s) term must beincluded if this expression is to uniquely represent the x coordinate ofa particular pixel when scanned in either direction.

The measured y coordinate corresponds to the location of the image ofthe d=1 detector on the image-containing medium. The image of the d'thdetector has a y coordinate of y+(d-1)δy where δy is the spacing betweenthe detector images. This expression also corresponds to the ycoordinate of the pixel measured by the d'th detector and is entered bythe microprocessor into the y holding register.

The expression to the right of the equal sign in equation (2) with V(read from the A/D converter 606 (FIG. 11)) substituted for V(p,d,c) iscomputed by the microprocessor and entered into the G holding register658 (FIG. 11). The numerator of the second term of the expressionV(p=0,d,c) is available in memory as a result of having performed the TAand SA interrupt routines (see FIGS. 14 and and the denominator is alsoavailable in memory as a result of having performed calculation 1002 ofthe calculate routine (see FIG. 20). The microprocessor enters the lightcolor c to which the pixel reflectivity/transmissivity G(p) correspondsinto the c holding register 662 (FIG. 11). The microprocessor enters thetype of image-containing medium being scanned, i.e. reflective ortransmissive, into the p output register 680 at the beginning of thereadout process.

After the data has been entered into the holding registers, themicroprocessor compares d with D 1114. If d does not equal D, themicroprocessor increments the d register 1118 and returns to the operateroutine to wait for the next SC interrupt. If d is equal to D, themicroprocessor compares c with 3 1122. If c does not equal 3, themicroprocessor performs the operations 1126 of entering 1 into the dregister and incrementing the c register. It then returns to the operateprogram and waits for the next SC interrupt. If d is equal to D and c isequal to 3, all of the detector array data has been read out. Themicroprocessor than disables the SC interrupts and returns to theoperate routine and waits for the next TC interrupt. The microprocessoralternately performs the TC interrupt routine followed by 3D SCinterrupt routines until the region of the image-containing medium forwhich data is desired has been completely scanned at which time themicroprocessor ends the scanning process by parking the optical assembly174 (FIGS. 2 and 3).

What is claimed is:
 1. A scanning apparatus for measuring thetransmissivity or reflectivity of an image-containing medium in twodimensions and converting said measurements to a digital format, saidimage-containing medium having a top surface and a bottom surface, theregion of space in contact with said top surface being above saidimage-containing medium, the region of space in contact with said bottomsurface being below said image-containing medium, the points in spacebeing identified by x, y, and z Cartesian coordinates, the x and ycoordinate axes being in the plane of said image-containing mediumbottom surface, the z axis being normal to said image-containing mediumplane, regions of said image-containing medium plane of a particularsize and shape and limited in extent in both x and y directionsconstituting pixels, the coordinates of each of said pixels being thoseof a locating interior point of each of said pixels, said locatinginterior points being similarly situated in all of said pixels, thepixels having the same x value constituting a column of pixels in saidimage-containing medium plane, the pixels having the same y valueconstituting a row of pixels in said image-containing medium plane, thereflectivity of a pixel being the ratio of the light reflected by thepixel to the light incident on the pixel, the transmissivity of a pixelbeing the ratio of the light emitted from a pixel to the light incidenton the pixel, said apparatus comprising:a means for supporting saidimage-containing medium; a means for detecting light, saidlight-detecting means comprising a plurality of light detectors, each ofsaid light detectors producing an electrical output that is a measure ofthe total light energy to which said light detector is exposed; a meansfor collecting the light emerging from a plurality of pixels anddirecting said collected light to said light detectors, said pixelsbeing imaged on said light detectors on a one-to-one basis; a means forpositioning said light-collecting means in an x-y plane, the position ofsaid light-collecting means being defined as the x and y coordinates ofa reference pixel, said reference pixel being the pixel that is imagedon a reference detector, said reference detector being one of said lightdetectors; a means for illuminating observed pixels, said observedpixels being those pixels from which light is collected by said lightcollecting means and directed to said light detectors, said observedpixels moving in said image-containing medium plane in concert with themovement of said light-collecting means; and a means for determining thex and y coordinates of said observed pixels, an optical means being usedin the determination of at least one of said coordinates.
 2. Thescanning apparatus of claim 1 wherein said positioning means comprises:ameans for transporting said light-collecting means along a straight lineparallel to said x axis; and a means for transporting saidlight-collecting means along a straight line parallel to said y axis. 3.The scanning apparatus of claim 2 wherein said y transporting meanstransports said x transporting means as well as said light-collectingmeans in the y direction.
 4. The scanning apparatus of claim 1 whereinsaid positioning means comprises a coarse positioning means and a finepositioning means, said coarse positioning means being capable ofpositioning said light-collecting means to within approximately 30pixels of a desired pixel, said fine positioning means being capable ofpositioning said light-collecting means at the desired pixel.
 5. Thescanning apparatus of claim 1 further comprising a means for tiltingsaid light-collecting means thereby permitting light to be collectedfrom a particular pixel within a group of pixels and directed to saidreference detector without translational movement of saidlight-collecting means by said positioning means.
 6. The scanningapparatus of claim 5 wherein said tilting means comprises:a tiltingplatform, said platform being mounted on said positioning means, saidlight-collecting means being mounted on said platform, said platformbeing U-shaped, the mounting surfaces of said platform being the outersides of the "U"; a piezoelectric device having first and secondparallel planar surfaces, said piezoelectric device being positionedwithin the opening of the "U", said first and second parallel planarsurfaces being in intimate contact with the inner sides of the "U"; ameans for applying a voltage to said piezoelectric device therebycausing the distance between said first and second surfaces of saidpiezoelectric device to change thereby causing said light-collectingmeans to tilt relative to said positioning means.
 7. The scanningapparatus of claim 1 wherein said illuminating means comprises a whitelight source.
 8. The scanning apparatus of claim 1 wherein saidilluminating means comprises at least one light source, each of saidlight sources producing light of a different color.
 9. The scanningapparatus of claim 8 wherein each of said light sources illuminatedifferent pixels at any given time.
 10. The scanning apparatus of claim8 wherein only one of said light sources is emitting light at any giventime.
 11. The scanning apparatus of claim 1 wherein each of said lightdetectors responds substantially only to light of a particular color.12. The scanning apparatus of claim 11 wherein said light detectors forma plurality of groups, each of said detector groups being substantiallyresponsive only to light of a particular color.
 13. The scanningapparatus of claim 1 wherein said light detectors are arranged in atleast one linear array, the line through the pixels imaged on thedetectors in each of said linear arrays being essentially parallel tosaid y axis.
 14. The scanning apparatus of claim 9 wherein said lightcollecting means comprises a lens, the length of the light path fromsaid pixels to said lens being the object distance, the length of thelight path from said lens to the images of said pixels being the imagedistance, said image distance being a function of the color of thelight, said light detectors being placed at positions for which theimage distances correspond to the color of light emanating from thepixels imaged on said light detectors.
 15. The scanning apparatus ofclaim 11 wherein said light collecting means comprises a lens, thelength of the light path from said pixels to said lens being the objectdistance, the length of the light path from said lens to the image of apixel being the image distance, said image distance being a function ofthe color of the light, said light detectors being placed at positionsfor which the image distances correspond to the color of light to whichthe respective light detectors are responsive.
 16. The scanningapparatus of claim 14 or claim 15 further comprising a means foradjusting said image distances to compensate for changes in objectdistance during scanning.
 17. The scanning apparatus of claim 16 whereinsaid image distance adjusting means causes the positions of said lightdetectors to be adjusted.
 18. The scanning apparatus of claim 17 whereinsaid image distance adjusting means comprises:a piezoelectric devicehaving first and second parallel planar surfaces, said light detectorsbeing mounted on said first surface, said second surface being fixedlypositioned with respect to said lens; a means for applying a voltage tosaid piezoelectric device thereby causing the distance between saidfirst and second surfaces of said piezoelectric device to change. 19.The scanning apparatus of claim 16 further comprising a means formeasuring said object distance.
 20. The scanning apparatus of claim 19wherein said medium supporting means comprises a transparent panelhaving a top and a bottom surface, the bottom surface of saidimage-containing medium being in intimate contact with the top surfaceof said transparent panel, said object distance measuring means beingfixedly positioned with respect to said lens, said object distancemeasuring means comprising:a means for generating a coherent planarlight wave, a means for splitting said light wave into a first lightwave and a second light wave; a means for directing said first lightwave to said transparent panel, said first light wave being partiallyreflected from said transparent panel and returning to said objectdistance measuring means; a reference mirror; a means for directing saidsecond light wave to said reference mirror, said second light wave beingreflected from said reference mirror and returning to said objectdistance measuring means; a means for combining said reflected firstlight wave and said reflected second light wave into a combination planewave, the amplitude of said combination plane wave being a function ofthe difference in phases of said reflected first and second waves at thepoint of said combining, the phases of said reflected first and secondlight waves being linearly related to first and second propagationtimes, said first propagation time being the time for said first lightwave to travel from said splitting means to said transparent panel andthen to said combining means, said second propagation time being thetime for said second light wave to travel from said splitting means tosaid reference mirror and then to said combining means; a means forvarying said second propagation time about an average value; a means forobtaining an electrical measure of the power of said combination planewave; a means for determining the changes in the difference in the phaseof said first reflected light wave and the average phase of said secondreflected light wave and for computing the algebraic sum of said changesfrom said power measure and said variations in said second propagationtime.
 21. The scanning apparatus of claim 20 wherein the region ofincidence of said first light wave on said transparent panel is directlybelow said observed pixels in said medium plane.
 22. The scanningapparatus of claim 21 further comprising a means for isolating saidpower measuring means from light emitted by said illuminating means. 23.The scanning apparatus of claim 21 further comprising a means forisolating said light-detecting means from light emitted by said coherentplanar light wave generating means.
 24. The scanning apparatus of claim22 wherein said isolating means includes a filter placed in the path ofsaid first light wave, said filter stopping a substantial portion oflight emitted by said illuminating means from reaching said powermeasuring means, said filter allowing light from said coherent planarlight wave generating means to pass through to said power measuringmeans.
 25. The scanning apparatus of claim 23 wherein said isolatingmeans includes a filter placed in the path of light collected by saidlight-collecting means, said filter stopping light emitted by saidcoherent planar light wave generating means from reaching saidlight-detecting means, said filter allowing a substantial portion oflight collected by said light-collecting means to pass through to saidlight-detecting means.
 26. The scanning apparatus of claim 22 whereinsaid isolating means includes a narrow-passband filter placed in thepath of said first light wave, said filter bandwidth being wide enoughto pass said first light wave, said filter bandwidth being narrow enoughto exclude a substantial portion of light from said illuminating means.27. The scanning apparatus of claim 23 wherein said isolating meansincludes a narrow-band rejection filter placed in the path of lightcollected by said light-collecting means, said filter bandwidth beingwide enough to exclude substantially all of the light from said coherentplanar light wave generating means, said bandwidth being narrow enoughto permit substantially all of the light collected by saidlight-collecting means to pass through to said light-detecting means.28. The scanning apparatus of claim 20 wherein said image distanceadjusting means translates said object distance measurements into imagedistance adjustments thereby maintaining said pixel images coincidentwith said light detectors.
 29. The scanning apparatus of claim 1 whereinsaid coordinate determining means comprises:a means for measuring the xand y coordinates of said reference pixel, an optical means being usedin the determination of at least one of said coordinates; a means fordetermining the x and y coordinates of all other observed pixelsutilizing the measured x and y coordinates of said reference pixel. 30.The scanning apparatus of claim 1 wherein the x and y coordinates ofeach of said other observed pixels is expressed respectively as the sumof the x coordinate of said reference pixel and a first linear functionof the x' and y' coordinates of said other pixel and the sum of the ycoordinate of said reference pixel and a second linear function of thex' and y' coordinates of said other pixel, the x'-y' coordinate systemhaving its origin at said reference pixel, the x'-y' coordinate axesbeing rotated from the x-y coordinate axes by an unknown angle, saidscanning apparatus further comprising a means for determining theconstants in said first and second linear functions, saidconstant-determining means including:an orientation reference located insaid medium plane adjacent to said image-containing medium, saidorientation reference being the line separating a region of highreflectivity and low transmissivity from a region of low reflectivityand high transmissivity.
 31. The scanning apparatus of claim 30 whereinsaid medium-supporting means comprises a transparent panel having a topand a bottom surface, the bottom surface of said medium being inintimate contact with the top surface of said transparent panel, saidorientation reference being the edge of a layer attached to the topsurface of said transparent panel, said layer having high reflectivityand low transmissivity, said transparent panel having low reflectivityand high transmissivity, the measured x and y coordinates of saidreference pixel corresponding to the coincidence of each of said otherobserved pixels, including said reference pixel, with said orientationreference providing the data for determining the constants in said firstand second linear functions.
 32. The scanning apparatus of claim 31wherein said orientation reference is the long edge of a rectangularstrip, said long edge being essentially parallel to said y axis, andsaid light detectors are arranged in at least one linear array, the linethrough the pixels imaged on said detectors in each of said lineararrays being essentially parallel to said y axis, saidconstant-determining means also including:a means for causing saidpositioning means to move parallel to said x axis across saidorientation reference, the light from an observed pixel changingsubstantially in magnitude when said observed pixel crosses saidorientation reference; a means for storing the coordinates of saidreference pixel the first time each of said light detectors yields asubstantial change in output as the region observed by saidlight-collecting means moves across said orientation reference; a meansfor calculating the constants in said first and second linear functions,said calculating means utilizing the data stored by said data storingmeans.
 33. The scanning apparatus of claim 29 wherein said optical meansfor measuring a coordinate comprises:a measuring scale havingoptically-readable coded markings along its length, each of said codedmarkings denoting distance along said scale, said scale being fixedlypositioned with respect to said medium-supporting means insofar asmovement along the longitudinal axis of said scale is concerned, saidlongitudinal axis being parallel to the axis of the coordinate beingmeasured; an optical means for reading said coded markings, said opticalreading means being fixedly positioned with respect to saidlight-collecting means insofar as movement along the longitudinal axisof said scale is concerned.
 34. The scanning apparatus of claim 29wherein said optical means for measuring a coordinate comprises:a planarstrip having a light-reflecting surface, said strip being fixedlypositioned with respect to said medium-supporting means, saidlight-reflecting surface being normal to the axis of the coordinatebeing measured, the long edge of said strip being normal to said z axis;a distance-measuring apparatus for measuring the distance to saidlight-reflecting strip, said apparatus fixedly positioned with respectto light-collecting means, said apparatus comprising: a means forgenerating a coherent planar light wave, a means for splitting saidlight wave into a first light wave and a second light wave; a means fordirecting said first light wave to said reflecting strip, said firstlight wave being reflected from said reflecting strip and returning tosaid distance-measuring apparatus; a reference mirror; a means fordirecting said second light wave to said reference mirror, said secondlight wave being reflected from said reference mirror and returning tosaid distance-measuring apparatus; a means for combining said reflectedfirst light wave and said reflected second light wave into a combinationplane wave, the amplitude of said combination plane wave being afunction of the difference in phases of said reflected first and secondwaves at the point of said combining, the phases of said reflected firstand second light waves being linearly related to first and secondpropagation times, said first propagation time being the time for saidfirst light wave to travel from said splitting means to said reflectingstrip and then to said combining means, said second propagation timebeing the time for said second light wave to travel from said splittingmeans to said reference mirror and then to said combining means; a meansfor varying said second propagation time about an average value; a meansfor obtaining an electrical measure of the power of said combinationplane wave; a means for determining the changes in the difference in thephase of said first reflected light wave and the average phase of saidsecond reflected light wave and for computing the algebraic sum of saidchanges from said power measure and said variations in said secondpropagation time.
 35. The scanning apparatus of claim 34 wherein saidmeans for varying said second propagation time in saiddistance-measuring apparatus comprises:a piezoelectric device havingfirst and second parallel planar surfaces, said reference mirror beingmounted on said first surface, said second surface being fixedlypositioned with respect to said combining means; a means for applying avoltage to said piezoelectric device thereby causing the distancebetween said first and second surfaces of said piezoelectric device tochange. a source of an alternating voltage, said alternating voltagebeing applied to said piezoelectric device.
 36. The scanning apparatusof claim 35 wherein said alternating voltage is a square wave ofpredetermined amplitude, said square wave causing the phase of saidsecond reflected wave at the point of combination with said firstreflected wave to shift back and forth between a first phase and asecond phase, said second phase being approximately one-quarterwavelength greater than said first phase, and said means for determiningthe changes in the difference in the phase of said first reflected lightwave and the average phase of said second reflected light wave and forcomputing the algebraic sum of said changes comprises:an electroniccircuit for converting said power measure into a first square wave and asecond square wave, the transitions between low and high values for saidfirst square wave coinciding with average value crossings of said powermeasure if the phase of said second reflected light wave were constantand equal to said first phase, the transitions between low and highvalues for said second square wave coinciding with average valuecrossings of said power measure if the phase of said second reflectedlight wave were constant and equal to said second phase; an electroniccircuit for counting low-to-high and high-to-low transitions of saidfirst square wave, said counting circuit counting upward when saidlow-to-high transition coincides with said second square wave being lowand when said high-to-low transition coincides with said second squarewave being high, said counting circuit counting downward when saidlow-to-high transition coincides with said second square wave being highand when said high-to-low transition coincides with said second squarewave being low, the positive (negative) count maintained by saidcounting circuit corresponding to the cumulative increase (decrease) inthe length of the path traveled by said first light wave from saidsplitting means to said reflecting strip and back to said combiningmeans measured in half-wavelength units.
 37. The scanning apparatus ofclaim 1 wherein said positioning means positions said light-collectingmeans utilizing measurements provided by said pixel coordinate measuringmeans.
 38. The scanning apparatus of claim 1 further comprising:a meansfor calibrating each of said light detectors.
 39. The scanning apparatusof claim 38 wherein said calibration means comprises:a means forexposing each of said light detectors to light of the same intensity forthe purpose of determining the responsivity of each of said detectors.40. The scanning apparatus of claim 39 wherein said calibrating meansfurther comprises:a means for measuring the dark current response ofeach of said light detectors.
 41. The scanning apparatus of claim 39wherein said medium supporting means comprises a transparent panelhaving a top and a bottom surface, the bottom surface of saidimage-containing medium being in intimate contact with the top surfaceof said transparent panel, said calibrating means comprising:a layer ofreflectivity calibrating material affixed to a region of the top surfaceof said transparent panel adjacent to the intended position of saidimage-containing medium, said reflectivity calibrating layer having atleast one region of uniform reflectivity, said uniform reflectivityregions being accessible to said positioning means for the collection oflight by said light-collecting means.
 42. The scanning apparatus ofclaim 39 wherein said medium supporting means comprises a transparentpanel having a top and a bottom surface, the bottom surface of saidimage-containing medium being in intimate contact with the top surfaceof said transparent panel, said calibrating means comprising:a layer oftransmissivity calibrating material affixed to a region of the topsurface of said transparent panel adjacent to the intended position ofsaid image-containing medium, said transmissivity calibrating layerhaving at least one region of uniform transmissivity, said uniformtransmissivity regions being accessible to said positioning means forthe collection of light by said light-collecting means.
 43. The scanningapparatus of claim 39 wherein said medium supporting means comprises atransparent panel having a top and a bottom surface, the bottom surfaceof said image-containing medium being in intimate contact with the topsurface of said transparent panel, said calibrating means comprising:alayer of reflectivity calibrating material affixed to a region of thetop surface of said transparent panel adjacent to the intended positionof said image-containing medium, said reflectivity calibrating layerhaving at least one region of uniform reflectivity, said uniformreflectivity regions being accessible to said positioning means for thecollection of light by said light-collecting means; a layer oftransmissivity calibrating material affixed to a region of the topsurface of said transparent panel adjacent to the intended position ofsaid image-containing medium, said transmissivity calibrating layerhaving at least one region of uniform transmissivity, said uniformtransmissivity regions being accessible to said positioning means forthe collection of light by said light-collecting means.
 44. The scanningapparatus of claim 40 wherein said dark current measuring means includesa means for shielding said light-detecting means from light.
 45. Thescanning apparatus of claim 44 wherein said means for shielding saidlight-detecting means from light is a pivoting solenoid-operated lightshield, the activation of said solenoid causing the light shield tocover said light-detecting means thereby preventing any light fromreaching said light-detecting means.
 46. The scanning apparatus of claim1 wherein said illuminating means illuminates said image-containingmedium from below.
 47. The scanning apparatus of claim 1 wherein saidilluminating means illuminates said image-containing medium from above.48. The scanning apparatus of claim 2 wherein said x transporting meanscomprises:a source of mechanical power; a lower x transporting meanscomprising a lower platform constrained to move below saidimage-containing medium plane along a straight line parallel to the xaxis and a means to couple said lower platform to said mechanical powersource for the purpose of moving said lower platform back and forthalong said straight line, said lower platform carrying saidlight-collecting means; an upper x transporting means comprising anupper platform constrained to move above said image-containing mediumplane along a straight line parallel to the x axis and a means to couplesaid upper platform to said mechanical power source for the purpose ofmoving said upper platform back and forth along said straight line, saidupper platform being positioned directly above said lower platform andconstrained to move in synchronism with said lower platform, said upperplatform carrying said illuminating means.
 49. The scanning apparatus ofclaim 2 wherein said illuminating means comprises a lower illuminatingmeans and an upper illuminating means, said lower illuminating meansilluminating said image-containing medium from below, said upperilluminating means illuminating said image-containing medium from above,said scanning apparatus further comprising a means for individuallyactivating said upper and lower illuminating means.
 50. The scanningapparatus of claim 49 wherein said x transporting means comprises:asource of mechanical power; a lower x transporting means comprising alower platform constrained to move along a straight line parallel to thex axis and a means to couple said lower platform to said mechanicalpower source for the purpose of moving said lower platform back andforth along said straight line, said lower platform carrying said lowerilluminating means and said light-collecting means; an upper xtransporting means comprising an upper platform constrained to movealong a straight line parallel to the x axis and a means to couple saidupper platform to said mechanical power source for the purpose of movingsaid upper platform back and forth along said straight line, said upperplatform being positioned directly above said lower platform andconstrained to move in synchronism with said lower platform, said upperplatform carrying said upper illuminating means.
 51. The scanningapparatus of claim 48 or claim 50 wherein said mechanical power sourceis a reversible motor having a rotatable shaft, and said lower platformcoupling means and said upper platform coupling means is a belt guidedalong a circuitous path by a plurality of pulleys, one of said pulleysbeing coupled to the rotatable shaft of said motor, said lower and upperplatforms being attached to said belt, the rotation of said motor shaftthereby causing said lower and upper platforms to move in synchronism,one above the other, along said straight line parallel to said x axis.52. The scanning apparatus of claim 51 wherein one of said pulleys is anidler pulley that exerts a predetermined force locally normal to saidbelt, said belt thereby being maintained in tension.
 53. The scanningapparatus of claim 1 further comprising:a means for controlling theoperations of said scanning apparatus.
 54. The scanning apparatus ofclaim 53 wherein said controlling means comprises:a means forcontrolling said positioning means, said position-controlling meanscausing said light-collecting means to sequentially occupy a pluralityof positions thereby causing light to be collected from all of thepixels in a predetermined region of said image-containing medium, theprocess of sequentially occupying said plurality of positionsconstituting a region scan.
 55. The scanning apparatus of claim 54wherein said predetermined region is rectangular having sides parallelto said x and y axes, each of said plurality of positions being denotedby said reference pixel coordinates x and y, said region scan beingaccomplished x scan by x scan, said x scan being the process by whichsaid light-collecting means occupies a sequence of positions on a lineparallel to said x axis, said x changing monotonically during each xscan, said y changing monotonically after each x scan.
 56. The scanningapparatus of claim 54 wherein said predetermined region is rectangularhaving sides parallel to said x and y axes, each of said plurality ofpositions being denoted by said reference pixel coordinates x and y,said region scan being accomplished x scan by x scan, said x scan beingthe process by which said light-collecting means occupies a sequence ofpositions on a line parallel to said x axis, said x increasingmonotonically during an x scan if x decreased monotonically during theprior x scan, said x decreasing monotonically during an x scan if xincreased monotonically during the prior x scan, said y changingmonotonically after each x scan.
 57. The scanning apparatus of claim 55or claim 56 wherein said position-controlling means causes thepositioning means to move at constant velocity while engaged in an xscan.
 58. The scanning apparatus of claim 55 or claim 56 wherein saidlight detectors are arranged in at least one linear array, the linethrough the pixels imaged on said detectors in each of said lineararrays being essentially parallel to said y axis, pixel light therebybeing collected for D pixel rows as a result of each x scan by saidpositioning means, D being the number of detectors in each of saidlinear arrays, the y coordinate of said positioning means being changedby M rows after each row scan, M being equal to or less than D, toensure measurement of all pixels in said region.
 59. The scanningapparatus of claim 58 wherein said controlling means further comprises ameans for storing each light detector output together with the x and ycoordinates of said reference pixel for all positions along an x scan,said storing means to have the capacity of storing said data for atleast one prior x scan, the light detector outputs obtained during acurrent row scan that duplicate results obtained during the prior rowscan being discarded.
 60. A method for measuring the transmissivity orreflectivity of an image-containing medium in two dimensions andconverting said measurements to a digital format, said image-containingmedium having a top surface and a bottom surface, the region of space incontact with said top surface being above said image-containing medium,the region of space in contact with said bottom surface being below saidimage-containing medium, the points in space being identified by x, y,and z Cartesian coordinates, the x and y coordinate axes being in theplane of said image-containing medium bottom surface, the z axis beingnormal to said image-containing medium plane, regions of saidimage-containing medium plane of a particular size and shape and limitedin extent in both x and y directions constituting pixels, thecoordinates of each of said pixels being those of a locating interiorpoint of said pixel, said locating interior points being similarlysituated in all of said pixels, the pixels having the same x valueconstituting a column of pixels in said image-containing medium plane,the pixels having the same y value constituting a row of pixels in saidimage-containing medium plane, the reflectivity of a pixel being theratio of the light reflected by the pixel to the light incident on thepixel, the transmissivity of a pixel being the ratio of the lightemitted from a pixel to the light incident on the pixel, said methodcomprising the steps:illuminating a plurality of pixels in an area ofsaid image-containing medium, the geometrical relationships among thepixels in said pixel area being predetermined, one of said pixelsserving as the reference pixel; measuring the intensity of the lightemerging from each of said pixels; measuring the x and y coordinates ofsaid reference pixel, at least one of said coordinates being obtained byoptically reading coded markings on a measuring scale; determining the xand y coordinates of all other pixels from the measurements of the x andy coordinates of said reference pixel.
 61. A method for measuring thetransmissivity or reflectivity of an image-containing medium in twodimensions and converting said measurements to a digital format, saidimage-containing medium having a top surface and a bottom surface, theregion of space in contact with said top surface being above saidimage-containing medium, region of space in contact with said bottomsurface being below said image-containing medium, the points in spacebeing identified by x, y, and z Cartesian coordinates, the x and ycoordinate axes being in the plane of said image-containing mediumbottom surface, the z axis being normal to said image-containing mediumplane, regions of said image-containing medium plane of a particularsize and shape and limited in extent in both x and y directionsconstituting pixels, the coordinates of each of said pixels being thoseof a locating interior point of said pixel, said locating interiorpoints being similarly situated in all of said pixels, the pixels havingthe same x value constituting a column of pixels in saidimage-containing medium plane, the pixels having the same y valueconstituting a now of pixels in said image-containing medium plane, thereflectivity of a pixel being the ratio of the light reflected by thepixel to the light incident on the pixel, the transmissivity of a pixelbeing the ratio of the light emitted from a pixel to the light incidenton the pixel, said method comprising the steps:illuminating a pluralityof pixels in an area of said image-containing medium, the geometricalrelationships among the pixels in said pixel area being predetermined,one of said pixels serving as the reference pixel; measuring theintensity of the light emerging from each of said pixels; measuring thex and y coordinates of said reference pixel, at least one of saidcoordinates being obtained by measuring the time required for a lightwave to travel from said reference pixel to the other coordinate axisand back; determining the x and y coordinates of all other pixels fromthe measurements of the x and y coordinates of said reference pixel. 62.The method of claim 60 or claim 61 wherein the determination of the xand y coordinates of all other pixels from the measurements of the x andy coordinates of said reference pixel comprises the steps:representingthe x coordinate of each of said other pixels as the sum of the xcoordinate of said reference pixel and a linear function of the x' andy' coordinates of said other pixel, the x'-y' coordinate system being inthe plane of said x-y coordinate system and having its origin at saidreference pixel, the x'-y' coordinate axes being rotated from the x-ycoordinate axes by an unknown angle; representing the y coordinate ofeach of said other pixels as the sum of the y coordinate of saidreference pixel and a linear function of the x' and y' coordinates ofsaid other pixel; placing an orientation reference for use indetermining the constants in said representations of the x and ycoordinates of said other pixel in said image-containing medium planeadjacent to said image-containing medium, said orientation referencebeing a straight line separating a region of high reflectivity and lowtransmissivity from a region of low reflectivity and hightransmissivity; measuring the x and y coordinates of said referencepixel when each of said pixels, including said reference pixel, is incoincidence with said orientation reference thereby providing the datafor determining the constants in said representations.
 63. The method ofclaim 61 wherein measuring the time required for a light wave to travelfrom said pixel region to the other coordinate axis and back comprisesthe steps:placing a reflecting surface parallel to said other axis;generating a coherent planar light wave; splitting said light wave intoa first light wave and a second light wave; directing said first lightwave to said reflecting surface, said first light wave being reflectedfrom said reflecting surface and returning; placing a reference mirrorwhere said second light wave can be reflected from said referencemirror; directing said second light wave to said reference mirror, saidsecond light wave being reflected from said reference mirror andreturning; combining said reflected first light wave and said reflectedsecond light wave into a combination plane wave, the amplitude of saidcombination plane wave being a function of the difference in phases ofsaid reflected first and second waves at the point of said combining,the phases of said reflected first and second light waves being linearlyrelated to first and second propagation times, said first propagationtime being the time for said first light wave to travel from the pointof said splitting to said reflecting surface and back to the point ofsaid combining, said second propagation time being the time for saidsecond light wave to travel from the point of said splitting to saidreference mirror and back to the point of said combining; varying saidsecond propagation time about an average value; obtaining an electricalmeasure of the power of said combination plane wave; determining thechanges in the difference in the phase of said first reflected lightwave and the average phase of said second reflected light wave andcomputing the algebraic sum of said changes from said power measure andsaid variations in said second propagation time.